Fuel Pressure regulator Words and Pictures: Rob Bell Grateful assistance from Tim Woolcott Click here for more on the FSE Power Boost Valve

Introduction: the Fuel Pressure regulator

Else where on this web site, we've discussed the various parts of the engine and where potential gains can be made by either modifying standard parts or replacing them with aftermarket items (air filters, throttle bodies, manifolds, exhaust systems etc).  One area that hasn't been touched upon is the fuel system. As part of your car's induction system, there is a fuel delivery system - the fuel rail - that takes the high pressure fuel pumped from the petrol tank by the fuel pump to the injectors. The fuel pressure, that to some extent determines the quantity of fuel delivered into the engine, is regulated by a simple pressure relief valve, also known as a Fuel Pressure Regulator (FPR). The FPRs on the standard car is 3.0 bar, plus or minus 0.2 bar - which means that one car's fuel rail pressure can vary as much as 14%. Ideally, if you have a high flow airfilter installed, along with other improvements to the engine's breathing and exhaust extraction system, you'd probably rather have a fuel pressure that is on the higher end of the scale rather than the lower end.

How can you tell whether your car's FPR rating is adequate for your car? Well, there probably isn't an easy answer to that unfortunately: a rolling road session with a lambda probe in the exhaust tract will give some useful guidance as to whether the engine is running lean at any part of it's acceleration cycle (the Lambda should be reading close to 1.0 for optimal stoichemetry of fuel and air). Another way of telling what fuel pressure your FPR is providing is to actually measure it with a pressure gauge inserted into the fuel line - but not everyone will have the suitable equipment to use. One on the road clue may be acceleration surge: if you accelerate hard past 4500 rpm, and partially lift the throttle and actually feel the car accelerating more, then this may be an indication that the engine is running somewhat lean at full throttle: consider an uprated FPR [See "UPDATE!" notes below].

What are the options available?

Two actually: the rising rate FSE Fuel Pressure 'boost' valve (pictured opposite - and more information and Thierry's rolling road data presented here), or a re-calibrated standard FPR. The former has a number of advantages that are claimed by the manufacturer's, but costs in the order of 100-150 UKP. In comparison, the re-calibrated standard FPR can be had for a fraction of this price, as all that is done is to 'squeeze' the FPR a little such that it holds a little more pressure...

I spoke to Mat Smith at http://www.msre.co.uk/ regarding the provision of a number of recalibrated FPRs - and at the time of writing, Mat still had some left (costing around 40 quid a unit). The remainder of this page covers the removal of the standard FPR, and the installation of the new, re-calibrated, FPR. I am hoping to get some comparative R.R. results to see what impact this had on my car at some point in the future. But impressions are pretty positive - particular from low engine speeds: where before the engine would tend to stutter a little when the accelerator was floored from idle rpm in traffic, the engine now pulls strongly and cleanly. Good power throughout the remainder of the rpm range - but significant increases in power aren't there - this isn't that sort of modification.

How to install a re-calibrated Fuel Pressure Regulator

1.	Location of FPR in fuel rail, as seen from above through engine inspection hatch Firstly you need to identify where the FPR is located - its on the right side of the engine, and is just about visible through the grille in the boot. To remove, you will need to remove the engine inspection cover (detailed here) so you can get access to both sides of the FPR.
2.	Next, you need to remove the fuel return line from the FPR (only a drop of petrol will fall out, although you may want to strategically position some old absorbent rags or tissue to catch any split fuel).
3.	Then you need to remove the retaining clip that holds the FPR to the fuel rail. It has two spring tabs that need to be pressed outwards (towards the rear of the car) before the retaining clip can be unscrewed. Shown opposite is the close up of the FPR and its retaining clip. As can be seen, it is held in place by two 'U' grips (one visible, the other on the opposite side), and the clip is prevented from moving by two 'spring tabs' - although this isn't all that clear on this picture, it is towards the right of the image (and another on the opposite side).
4.	The next stage is the trickiest part, as both spring tabs need to be pushed towards the rear of the car so that the retaining clip can be removed - this is shown in both the images opposite:
5.	Once this has been successfully completed (easier said than done, as you'll find out, especially if you have a car with a VVC style alloy inlet plenum!), rotate the retaining clip and remove.
6.	Now you can simply withdraw the old FPR from the fuel rail hosing. You may find that the rubber sealing 'O' rings come out with the FPR (these are the brown rings shown on the FPR opposite). If so, remove these from the FPR, and re-insert them into the fuel rail FPR housing before pushing in the new FPR.
7.	New, re-calibrated FPR behind the old, original FPR in foreground Because you are effectively replacing 'like with like', reassembly is incredibly simple: simply push the new, re-calibrated, FPR into the Fuel rail housing, offer up the retaining clip, rotate it to lock into position, reattach the fuel return pipe to the FPR, and the job's a good 'un!

What are the driving impressions?

The installation of the up-regulated FPR proved to be surprisingly successful - although I asked Mat to manufacture these, I wasn't 100% convinced that it would have a dramatic effect upon power delivery - it was one of those things: I was curious, and the mod cheap enough to venture to have a go. So what did I find? To my surprise, I found that the initial throttle response was much better with the new FPR installed. It's difficult to quantify, but it was there. More noticeable was that any faltering in engine speed that was present when drive was established from low engine speeds in low gear (i.e. pulling away in 1st gear) was now completely absent. The car honestly pulled like a train!!! There weren't many 'flat spots' in my car's power curve (these had been mostly eradicated by the K&N in the airfilter enclosure modification), but this new FPR does seem to make the torque curve stronger - I think that I'll have to wait and see the torque curve before definitively commenting on that. But more performance? I am not sure about that. Maybe. What is more noticeable now is that there seems to be a breathing restriction at 4500-5000rpm - this is something that was always present, and perhaps the new FPR has simply thrown this into sharper relief, I am not sure. But this is an area that I want to investigate further as I have some ideas about how to over come this restriction...

Conclusions?

If you are asking me whether I'd recommend this modification, then yes I would. It's cheap (I think that Mat is asking 45 quid for these), simple to fit (at least into the MPi cars, if not necessarily the VVC), and seems to have a noticeable benefit in terms of driveability. And that alone is enough to justify this modification. That it also uses a completely standard part, and uses effectively a 'standard' fuel pressure rating which makes it 'invisible' in terms of warranty, is just icing on the cake

UPDATE! - 10th July 2004

I've recently had an opportunity to return to G-Force's excellent rolling road facility in Aylesbury to check out the power delivery and fuelling from the up-rated FPR - something that I had intended on doing from the outset (indeed, I was going to fit the FPR at the RR but a leaking inlet manifold gasket thwarted those particular efforts). In fact, it is very interesting to read back on my original notes mentioned above...

In fact, the seat of the pants feel of the car proved to be surprising accurate: the 3.3bar FPR had indeed bolstered the torque curve throughout most of the rev range BUT the power curve flattens off dramatically beyond 5000rpm...

If you click on the above image, you can enlarge it to get more detail - but comparing my previous power curve with the one following the installation of the 3.3bar FPR, one sees a dramatic attenuation of top end power. And there was me wondering why I was 2 seconds slower at Goodwood this year compared to last: I'd lost 10bhp!!! By the way, if you think that close to 145 with a standard K-series seems a little on the high side, then you're absolutely right: G-Force's flywheel power estimation algorithm is for a Porsche 911 - and not an MGF! The figure is actually rather closer to 133bhp for my car based on Emerald RR data - but that's a discussion for another day; the thing we're wanting to look at here is power changes - and we've certainly got that!

The reason for this power loss became very apparent when the fuelling stoichiometry was measured (in this case, in the form of air:fuel ratio - or APR). Peak power on most cars lies with an APR between 12.0 and 13.5 - and experience with K-series engines, this peak figure seems to coincide with 12.8-13.1. Now study the APR/ Power trace below:

Startlingly, the APR is hopelessly rich, dropping off the bottom of the scale beyond 5000 rpm. Little wonder that peak power had been diminished! At this level of over-fuelling, bore-wash (where unburnt petrol washes down the sides of the pistons, taking lubricating oil with it) is a serious risk. This is a very similar experience to that had by Thierry with his FSE FPR (click here): too much fuel which displaces air resulting in an inefficient burn of the fuel... and LESS POWER!

Needless to say that I've now reverted back to the original FPR. I'll be checking the fuelling once again when we return to the rolling road again later in the year.

Conclusions?

The bottom line is that the standard fuel pressure regulator at 3.0bar is very likely to ideally suit your engine if it is essentially standard - airfilters and exhausts not withstanding. BUT if you've got a ported cylinder head, or are running hotter cams, then an up-rated FPR starts to make sense - but I would very much recommend you go to a RR to get the FPR properly set up to gain the most from it.